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      </div>
    
  </div>


  
  <h1 id="第二章-C语言概述"><a href="#第二章-C语言概述" class="headerlink" title="第二章 C语言概述"></a>第二章 C语言概述</h1><h2 id="2-1简单的C程序示例"><a href="#2-1简单的C程序示例" class="headerlink" title="2.1简单的C程序示例"></a><a href="#2.1">2.1简单的C程序示例</a></h2><h2 id="2-2-示例解释"><a href="#2-2-示例解释" class="headerlink" title="2.2 示例解释"></a><a href="#2.2">2.2 示例解释</a></h2><h3 id="2-2-1-第1遍：快速概要"><a href="#2-2-1-第1遍：快速概要" class="headerlink" title="2.2.1 第1遍：快速概要"></a><a href="#2.2.1">2.2.1 第1遍：快速概要</a></h3><h3 id="2-2-2-第2遍：程序细节"><a href="#2-2-2-第2遍：程序细节" class="headerlink" title="2.2.2 第2遍：程序细节"></a><a href="#2.2.2">2.2.2 第2遍：程序细节</a></h3><h2 id="2-3-简单的程序结构"><a href="#2-3-简单的程序结构" class="headerlink" title="2.3 简单的程序结构"></a><a href="#2.3">2.3 简单的程序结构</a></h2><h2 id="2-4-提高程序可读性的技巧"><a href="#2-4-提高程序可读性的技巧" class="headerlink" title="2.4 提高程序可读性的技巧"></a><a href="#2.4">2.4 提高程序可读性的技巧</a></h2><h2 id="2-5-进一步使用C语言"><a href="#2-5-进一步使用C语言" class="headerlink" title="2.5 进一步使用C语言"></a><a href="#2.5">2.5 进一步使用C语言</a></h2><h3 id="2-5-1-程序说明"><a href="#2-5-1-程序说明" class="headerlink" title="2.5.1 程序说明"></a>2.5.1 程序说明</h3><h3 id="2-5-2-多条声明"><a href="#2-5-2-多条声明" class="headerlink" title="2.5.2 多条声明"></a>2.5.2 多条声明</h3><h3 id="2-5-3-乘法"><a href="#2-5-3-乘法" class="headerlink" title="2.5.3 乘法"></a>2.5.3 乘法</h3><h3 id="2-5-4-打印多个值"><a href="#2-5-4-打印多个值" class="headerlink" title="2.5.4 打印多个值"></a>2.5.4 打印多个值</h3><h2 id="2-6-多个函数"><a href="#2-6-多个函数" class="headerlink" title="2.6 多个函数"></a><a href="#2.6">2.6 多个函数</a></h2><h2 id="2-7-调试程序"><a href="#2-7-调试程序" class="headerlink" title="2.7 调试程序"></a><a href="#2.7">2.7 调试程序</a></h2><h3 id="2-7-1-语法错误"><a href="#2-7-1-语法错误" class="headerlink" title="2.7.1 语法错误"></a>2.7.1 语法错误</h3><h3 id="2-7-2-语义错误"><a href="#2-7-2-语义错误" class="headerlink" title="2.7.2 语义错误"></a>2.7.2 语义错误</h3><h3 id="2-7-3-程序状态"><a href="#2-7-3-程序状态" class="headerlink" title="2.7.3 程序状态"></a>2.7.3 程序状态</h3><h2 id="2-8-关键字和保留标识符"><a href="#2-8-关键字和保留标识符" class="headerlink" title="2.8 关键字和保留标识符"></a><a href="#2.8">2.8 关键字和保留标识符</a></h2><h2 id="2-9-关键概念"><a href="#2-9-关键概念" class="headerlink" title="2.9 关键概念"></a><a href="#2.9">2.9 关键概念</a></h2><h2 id="2-10-本章小结"><a href="#2-10-本章小结" class="headerlink" title="2.10 本章小结"></a><a href="#2.10">2.10 本章小结</a></h2><h2 id="2-11-复习题"><a href="#2-11-复习题" class="headerlink" title="2.11 复习题"></a><a href="#2.11">2.11 复习题</a></h2><h2 id="2-12-编程练习"><a href="#2-12-编程练习" class="headerlink" title="2.12 编程练习"></a><a href="#2.12">2.12 编程练习</a></h2><hr>
<p><strong><a name="2.1">2.1简单的C程序示例</a></strong></p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta"># <span class="meta-keyword">include</span> <span class="meta-string">&lt;stdio.h&gt;</span></span></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">(<span class="keyword">void</span>)</span></span>&#123;   <span class="comment">/* a simple code */</span></span><br><span class="line">	<span class="keyword">int</span> num; <span class="comment">/* define a var call num */</span></span><br><span class="line">	num = <span class="number">1</span>; <span class="comment">/* get a value to num */</span></span><br><span class="line">	</span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">&quot;I am a simple &quot;</span>);</span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">&quot;computer.\n&quot;</span>);</span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">&quot;My favorite number is %d because it is first.\n&quot;</span>,num);</span><br><span class="line">	</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125; </span><br></pre></td></tr></table></figure>

<p>result</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">I am a simple computer.</span><br><span class="line">My favorite number is 1 because it is first.</span><br></pre></td></tr></table></figure>

<p><strong><a name="2.2">2.2 示例解释</a></strong></p>
<ul>
<li>第1遍：快速概要</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"># include &lt;stdio.h&gt; 包含另一个文件</span><br></pre></td></tr></table></figure>

<p>改行告诉编译器把<strong>stdio.h</strong>中的内容包含在当前程序中。stdio.h 是C编译器软件包的标准部分，它提供键盘输入和屏幕输出的支持。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">int main(void) 函数名</span><br></pre></td></tr></table></figure>

<p>C程序包含一个或多个函数，圆括号表明main()是一个函数名，int表明main()函数返回一个整数，void表明main()不带任何参数。其中int 和void 是ANSI C定义的main() 的一部分（如果使用其他的编译器，可能要忽略void）</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">&#x2F;*一个简单的C程序 这里是注释*&#x2F;</span><br><span class="line">&#123;	函数体开始</span><br><span class="line">&#125; 	函数体结束</span><br><span class="line">num &#x3D; 1; &#x2F;&#x2F; 赋值表达式</span><br><span class="line">printf(&quot; I am a simple &quot;); &#x2F;&#x2F;调用一个printf标准C函数</span><br><span class="line">printf(&quot;computer.\n&quot;); &#x2F;&#x2F; 其中 \n 是换行符</span><br><span class="line">printf(&quot;My favorite number is %d because it is first.\n&quot;,num); </span><br><span class="line">&#x2F;&#x2F; 这里将num的值内嵌在双引号括起来的内容中一并打印， %d 告诉计算机以何种形式输出num 的值，打印在何处。</span><br><span class="line">return 0;</span><br><span class="line">&#x2F;&#x2F; C 函数可以给调用方法提供或返回一个数</span><br></pre></td></tr></table></figure>

<ul>
<li>第2遍：程序细节</li>
</ul>
<ol>
<li><p>include &lt;stdio.h&gt; 的作用相当于将 stdio.h 文件中的所有内容都输入该行所在位置。实际上这是一种 “拷贝-粘贴” 的操作。<br>include 文件提供了一种方便的途径共享许多程序共有的信息。</p>
<p>include 这行代码是一条C预处理器指令。通常C编译前会对源码进行预处理。<br>所有的C编译软件包都会提供 stdio.h 文件，这是标准的输入输出头文件 standard input output 。通常在C程序顶部的<strong>信息集合</strong>被 称为头文件(header)。</p>
<p>大多数情况下，头文件包含了编译器创建最终可执行程序要用到的信息。例如，头文件中可以定义一些常量，或者指明函数名以及如何使用它们。但是，函数的实际代码在一个预编译的库文件中。头文件帮助编译器把编写的程序组合在一起。<br>如果不使用printf()函数可以不包含其头文件，但是不推荐这样做。C语言不内置输入输出函数是为了更加灵活的应对不同需求，例如编写汽车燃油系统或者蓝光播放芯片就不需要输入和输出函数。</p>
<p>include 的 # 符号表示，C预处理器在编译器接手之前处理这条指令。</p>
</li>
<li><p>main() 函数</p>
<p>main 是一个极其普通的名称，相对于master ，对于C语言来说这是唯一的选择。C 程序一定从 main() 函数开始执行。圆括号()用于识别main() 是一个函数。<br>int 是 main() 函数的返回类型。这表明 main() 函数返回的值是整数，返回给操作系统。<br>一些老的C代码可能就只有一个 mian() 函数，这是C90才接受了，C99和C11会让它爪巴。<br>void main() 这种是所有标准都不认可的写法，至于其出处就不多提了，免得白给，移植代码时坚持按照标准形式就不会出现问题。</p>
</li>
<li><p>注释</p>
<p>在程序中被 /**/ 括起来的内容就是程序的注释，C99 新加了一种风格的注释，普遍适用C++和JAVA。就是 // 单行注释。但是有一些编译器可能不支持这一特性，或者需要设置才能使用。</p>
</li>
<li><p>花括号、函数体和块</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">&#123;</span><br><span class="line">	...</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>花括号将函数体括起来，不可以省略。</p>
</li>
<li><p>声明</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">int num; &#x2F;&#x2F; 声明</span><br></pre></td></tr></table></figure>

<p>声明是C语言最重要的特性之一，以上声明完成了两个事情，一个是在函数中有一个名为num的变量。另一个是int表明num是一个整数。编译器用这些信息给num变量分配内存空间来存储。分号是C语言中大部分语句和声明的一部分。<br>int是C源语言中的一个关键字，表示一种基本的数据类型，关键字不可以用作其他用途。实例中的 num 是一个标识符，是一个变量、函数、或其它实体的名称。声明 就是将<strong>标识符</strong>与<strong>计算机内存中的特定位置</strong>联系起来。<br>在语言中所有变量必须先声明才能使用，在以前的C语言还要求把所有的变量要声明在块的顶部，其它语句不可以出现在任何声明的前面，例如：</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span>&#123;</span><br><span class="line">	<span class="keyword">int</span> doors;</span><br><span class="line">	<span class="keyword">int</span> dogs;</span><br><span class="line">	doors = <span class="number">5</span>;</span><br><span class="line">	dogs = <span class="number">3</span>;</span><br><span class="line">	<span class="comment">//其它语句</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>C99 和C11 遵循C++的惯例，可以将声明放在任何位置，但是在使用前也是要声明变量的。</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span>&#123;</span><br><span class="line">	<span class="keyword">int</span> doors;</span><br><span class="line">	doors = <span class="number">5</span>;</span><br><span class="line">    <span class="comment">//其它语句</span></span><br><span class="line">    <span class="keyword">int</span> dogs;</span><br><span class="line">	dogs = <span class="number">3</span>;</span><br><span class="line">	<span class="comment">//其它语句</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>为了和旧系统更好的兼容，尽量使用第一种声明方式。</p>
<ul>
<li>数据类型 有整数、字符、浮点数。把变量声明为整型或字符类型，计算机才能正确的存储、读取和解释数据。</li>
<li>命名 给变量命名要使用有意义的变量名或标识符，可以辅助注释进一步说明，这是一种良好的编程习惯，因为注释也是编程的一部分。C99 和C11 允许使用更长的标识符名，但是编译器只识别前63个字符。对于外部标识符只允许使用31个字符。以前的C90只允许6个字符。可以使用小写大写字母、数字、下划线_命名，且名称的第一个字符必须是字母或下划线 _</li>
<li>C99之前的标准是将所有声明都置于块的顶部，好处是便于理解程序的用途；C99允许在需要时才声明变量，其好处是不会忘记给变量赋值。但是实际上，许多编译器还不支持C99。</li>
</ul>
</li>
<li><p>赋值</p>
<p>num = 1 ; </p>
<p>在声明变量前编译器在计算机内存中为变量num预留了空间，然后执行赋值表达式（C语言是通过赋值运算符而不是赋值语句完成赋值操作，根据C标准，C语言没有赋值语句，本书或其他书提到的<strong>赋值语句</strong>指的是<strong>表达式语句</strong> - C语言基本语句之一）将值存储在之前预留的位置。</p>
</li>
<li><p>printf()函数</p>
</li>
<li><p>return 语句</p>
<p>C标准要求C 的main()要有return 语句。但是如果程序遗漏了return 语句，程序在运行最后至最外面的右花括号时也会返回0，可以省略但是不推荐。</p>
<p><strong><a name="2.3">2.3 简单的程序结构</a></strong></p>
</li>
</ol>
<p>程序由一个或多个函数组成，必须有main()函数，函数由函数头和函数体组成。函数头包括函数名、传入参数和返回类型。</p>
<p> <strong><a name="2.4">2.4 提高程序可读性的技巧</a></strong></p>
<ol>
<li><p>选择有意义的函数名和写注释，但是避免重复，有些变量名可以直接看出意思就不用注释。</p>
</li>
<li><p>善用空格，语句分行。</p>
<p><strong><a name="2.5">2.5 进一步使用C语言</a></strong></p>
</li>
</ol>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// fathm_ft.c -- 把2英寻转化为英尺 </span></span><br><span class="line"><span class="meta"># <span class="meta-keyword">include</span> <span class="meta-string">&lt;stdio.h&gt;</span></span></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">(<span class="keyword">void</span>)</span></span></span><br><span class="line"><span class="function"></span>&#123;  </span><br><span class="line">	<span class="keyword">int</span> feet,fathoms; </span><br><span class="line">	</span><br><span class="line">	fathoms = <span class="number">2</span>;</span><br><span class="line">	feet = <span class="number">6</span> * fathoms;</span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">&quot;There are %d feet in %d fathoms!\n&quot;</span>,feet,fathoms);</span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">&quot;Yes,I said %d feet!\n&quot;</span>,<span class="number">6</span>*fathoms);</span><br><span class="line">	</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125; </span><br></pre></td></tr></table></figure>

<p>result</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">There are 12 feet in 2 fathoms!</span><br><span class="line">Yes,I said 12 feet!</span><br></pre></td></tr></table></figure>

<ul>
<li><p>程序说明</p>
<p>程序开始有一条注释，给出了<strong>文件名和程序目的</strong>，对于浏览程序帮助很大。</p>
</li>
<li><p>多条声明</p>
<p>可以在一条语句中用逗号分隔来声明多个变量。</p>
</li>
<li><p>乘法</p>
<p>‘*’ 表示乘法运算</p>
</li>
<li><p>打印多个值</p>
<p>printf 待打印的值不一定是变量，只要是可以求出合适类型的值的项即可。</p>
<p><strong><a name="2.6">2.6 多个函数</a></strong></p>
</li>
</ul>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/* two_func.c -- 一个文件中包含两个函数 */</span></span><br><span class="line"><span class="meta"># <span class="meta-keyword">include</span> <span class="meta-string">&lt;stdio.h&gt;</span></span></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">butler</span><span class="params">(<span class="keyword">void</span>)</span></span>; <span class="comment">/* ANSI/ISO C 函数类型*/</span> </span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">(<span class="keyword">void</span>)</span></span>&#123;   </span><br><span class="line">	</span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">&quot;I will summon the butler function.\n&quot;</span>);</span><br><span class="line">	butler();</span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">&quot;Yes,Bring me some tea and writeable DVDS.\n&quot;</span>);</span><br><span class="line">	</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125; </span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">butler</span><span class="params">(<span class="keyword">void</span>)</span></span>&#123;	<span class="comment">/* 函数定义开始 */</span></span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">&quot;You rang,sir?\n&quot;</span>); </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>result:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">I will summon the butler function.</span><br><span class="line">You rang,sir?</span><br><span class="line">Yes,Bring me some tea and writeable DVDS.</span><br></pre></td></tr></table></figure>

<p>butler() 函数在程序中出现3次，第一次是函数原型，告知编译器在程序中要使用该函数；第二次是以函数调用的形式出现，最后一次是以函数定义出现，函数定义即函数的源码。<br>函数原型也称为函数声明，其指明了函数的属性，第一个void表示其函数的返回值类型，butler()函数没有返回值，第二个void表示函数不带参数，void指的是空的，不是无效。<br>早期的C支持不用描述参数的函数声明,虽然现在的C90、C99、C11也支持这种写法，但是不推荐。</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">butler</span><span class="params">()</span></span>; </span><br></pre></td></tr></table></figure>

<p>C的惯例是将main()函数放在开头，所以便将其他要用的函数先声明后续再在main()后面写出其函数定义。<br>头文件 stdio.h 文件包含了 printf() 函数原型。</p>
<p> <strong><a name="2.7">2.7 调试程序</a></strong></p>
<ul>
<li><p>语法错误</p>
</li>
<li><p>语义错误</p>
</li>
<li><p>程序状态</p>
<p>debug 下断点调试 查看对应变量</p>
<p><strong><a name="2.8">2.8 关键字和保留标识符</a></strong></p>
</li>
</ul>
<p><img src="https://gitee.com/hardychenlong/giteeblogimg/raw/master/20201016191353.png" class="lazyload" data-srcset="https://gitee.com/hardychenlong/giteeblogimg/raw/master/20201016191353.png" srcset="" alt="image-20201016191346275"></p>
<p><strong><a name="2.9">2.9 关键概念</a></strong></p>
<p>作为程序员，应该在符合C标准的编译器框架中，表达希望程序应该如何完成任务的想法。</p>
<p><strong><a name="2.10">2.10 本章小结</a></strong></p>
<p>C语言由一个或多个函数组成，每个C函数必须包含一个main()函数，C语言中的大部分语句以分号；结尾。声明语句为变量指定变量名，变量名是一种标识符，让计算机知道其存储的类型和所消耗内存大小。</p>
<p><strong><a name="2.11">2.11 复习题</a></strong></p>
<ol>
<li><p>C语言的基本模块是什么？</p>
<p>C语言的基本模块是函数。</p>
</li>
<li><p>语法错误 ： 不符合C语言的语法规范而引起的错误</p>
</li>
<li><p>语义错误 ： 逻辑错误，语法正确，但是功能和逻辑表达上错误。</p>
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